The present invention relates to well drilling operations, and particularly, to forming fissures in a subterranean target zone.
The production rate of a hydrocarbon producing well is for the most part directly related to the surface or contact area formed within the hydrocarbon bearing horizon through the process of drilling. Additionally, the smaller the contact area the more likely there will be excessive hydrocarbon flow rates in the target zone that will likely force sand or other residue into the flow path, thus creating obstacles and potentially clogging and slowing the hydrocarbon production flow. For example, residue may be forced into narrow fractures, well casings, and other production equipment, blocking the free flow of hydrocarbons.
Hydrocarbon bearing horizons occur as horizontal or subhorizontal layers of varying shapes and thicknesses known as traps. The depths below the surface of the earth at which these traps occur vary widely from a few hundred to thousands of feet. Increasing the contact area in such horizons is typically achieved by (1) creating fractures in the rock by hydro-fracturing, (2) using directional drilling techniques to maximize the length of the bore extending into the reservoir, for example, by redirecting the bore to a horizontal or subhorizontal orientation within the horizon, (3) drilling multiple lateral bores that deviate or extend from the main bore and into multiple target zones within the horizon.
Well bores are generally drilled with rotary rock cutting bits using a mix of water and mud or using compressed air to remove residue generated by the drilling process. The typical bore diameter of the bit used for penetrating the horizons of its hydrocarbon producing potential is 6⅞″ in diameter. Drilling larger diameter bores to the depth of the target zone within the hydrocarbon bearing horizon is generally cost prohibitive; therefore, the contact area, which is determined by the surface area of the cylinder defined by the bore, is somewhat limited and expensive to increase when achieved by well bores alone.
Creating fractures within the horizon in order to increase the contact area is typically achieved by using pressure, for example, by pumping large volumes of water or other fluids into the target zone of the horizon, a process called hydro-fracturing. Although this typical fracturing technique creates fissures that increase the contact area, it is difficult to accurately predict or control the plane through which the fissure is created and the expanse of the fissure.
For rock formations at a depth of less than 2000 feet, the fracturing generally extends in a substantially horizontal plane, whereas as for formations at depths greater than 2000 feet, the dominant fractures are vertical. In some formations fractures are the only porosity available for hydrocarbon flow to the well bore. Depending on the geological characteristics of the reservoir (target zone), the resulting fissure may not extend to the desired span, may be in the same plane as natural fissures and therefore not intersect them, or the fissure may extend beyond the target zone and into material other than the target zone. If this zone is water bearing, the fracturing process has the potential for making the well unsuitable for further production. Some rock types may not be suitable for using conventional methods of fracturing, for example shales. It is also difficult to control the thickness of fissures formed by hydro-fracturing or other conventional techniques, thus limiting the ability to control clogging of the fissures. Hydraulically non-uniform features, clogging, or other production problems in the well relating to fracturing can be costly problems to overcome, if they can be overcome at all.